Temporal Relations: Reference or Discourse Coherence?

TEMPORAL RELATIONS: REFERENCE OR DISCOURSE COHERENCE? Andrew Kehler Harvard University Aiken Computation Laboratory 33 Oxford Street Cambridge, MA 02138 kehler@das.harvard.edu Abstract The temporal relations that hold between events de- scribed by successive utterances are often left implicit or underspecified. We address the role of two phenom- ena with respect to the recovery of these relations: (1) the referential properties of tense, and (2) the role of temporal constraints imposed by coherence relations. We account for several facets of the identification of temporal relations through an integration of these. Introduction Tense interpretation has received much attention in lin- guistics (Partee, 1984; Hinrichs, 1986; Nerbonne, 1986, inter alia) and natural language processing (Webber, 1988; Kameyama et al., 1993; Lascarides and Asher, 1993, inter alia). Several researchers (Partee, 1984; Hinrichs, 1986; Nerbonne, 1986; Webber, 1988) have sought to explain the temporal relations induced by tense by treating it as anaphoric, drawing on Reichen- bach's separation between event, speech, and reference times (Reichenbach, 1947). Specifically, to account for the forward progression of time induced by successive simple past tenses in a narrative, they treat the simple past as referring to a time evoked by a previous past tense. For instance, in Hinrichs's (1986) proposal, ac- complishments and achievements x introduce a new ref- erence point that is temporally ordered after the time of the event itself, "ensuring that two consecutive ac- complishments or achievements in a discourse are al- ways ordered in a temporal sequence." On the other hand, Lascarides and Asher (1993) take the view that temporal relations are resolved purely as a by-product of reasoning about coherence relations holding between utterances, and in doing so, argue that treating sim- ple and complex tenses as anaphoric is unnecessary. This approach parallels the treatment of pronoun res- olution espoused by Hobbs (1979), in which pronouns are modeled as free variables that are bound as a by- product of coherence resolution. The Temporal Cen- tering framework (Kameyama et al., 1993) integrates lWe will limit the scope of this paper by restricting the discussion to accomplishments and achievements. aspects of both approaches, but patterns with the first in treating tense as anaphoric. We argue that aspects of both analyses are necessary to account for the recovery of temporal relations. To demonstrate our approach we will address the following examples; passages (la-b) are taken from Lascarides and Asher (1993): (1) a. Max slipped. He spilt a bucket of water. b. Max slipped. He had spilt a bucket of water. c. Max slipped because he spilt a bucket of water. d. Max slipped because he had spilt a bucket of water. Passage (la) is understood as a narrative, indicating that the spilling was subsequent to the slipping. Pas- sages (lb-d) are instead understood as the second clause explaining the first, indicating that the reverse temporal ordering holds. We address two related questions; the first arises from treating the simple past as anaphoric. Specifically, if a treatment such as Hinrichs's is used to explain the forward progression of time …

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